UnitCell

ABINIT needs three dimensioned non-coplanar vectors, forming the unit cell, to
set up the real space lattice.

An initial set of three vectors, specified in real space by rprim or as
unit vectors with angles angdeg, are dimensioned in a second step using
scaling factors as specified by acell or by rescaling their cartesian
coordinates, as specified by scalecart. Internally, only the final result,
%rprimd matters. The most detailed explanation can be found by looking at
rprim.

Note that ABINIT expects the mixed product of the three vectors (R1xR2).R3 to
be positive. If it is not the case, exchange two of them.

More information about the way the real space lattice, the reciprocal lattice,
and symmetries are defined in ABINIT can be found here.

Smart symmetriser

ABINIT has also a smart symmetriser capability, when spgroup!=0 and
brvltt=-1. In this case, the CONVENTIONAL unit cell must be input through
the usual input variables rprim, angdeg, acell and/or
scalecart. ABINIT will fold the conventional unit to the primitive cell,
and also generate all the nuclei positions from the irreducible ones. See
topic_SmartSymm.